Hydrocarbon wells, for example subsea oil wells, require a pipeline through which the hydrocarbon fluid is transported. In the case of subsea wells, these may be of substantial length, sometimes several tens of kilometres long. Consequently, the pipeline is a major cost element of the fluid extraction system. Many subsea wells have to contend with very high fluid pressures, for example as high as 700 bar. For a pipeline to withstand such pressure would require it to have a substantial wall thickness, and it is not cost effective to implement this for long pipelines. To decrease cost, it is preferable instead to reduce the maximum operating pressure of the pipeline, typically to about 200 bar, using devices such as valves or chokes to reduce the fluid pressure from the well. However, a failure of the pressure reducing device may result in overpressure in the pipeline with disastrous results. To prevent this happening, a pipeline protection system is typically incorporated into the pipeline proximate the well. Such a system must have high integrity and virtually guarantee to prevent any overpressure from the well from reaching the pipeline.
FIG. 1 shows a typical conventional high integrity pipeline protection system (HIPPS). Two hydraulically operated ‘HIPPS’ barrier valves 1 and 2 are inserted in the fluid extraction flow line 3, prior to the thinner walled pipeline 4. These valves are opened when their operating hydraulic cylinders are fed with hydraulic pressure, and closed, typically under spring pressure, when the hydraulic pressure is removed and vented, i.e. they are failsafe. Pressure transducers 5, 6 and 7 are fitted between the valves 1 and 2. The pressure transducers 5, 6 and 7 are connected to a subsea control module (SCM) 8, which houses a hard-wired and thus also high integrity, electronic safety critical control board. This board produces an output that energises directional control valves (DCVs) which in turn operate the valves 1 and 2. The DCVs are also failsafe in that they close the hydraulic pressure source, and open the valve 1 and 2 actuating cylinders to vent when de-energised. Thus loss of electrical or hydraulic power causes the valves 1 and 2 to close. The safety critical control board in the SEM 8 contains logic that de-energises the DCVs if two out of the three transducers 5, 6 and 7 indicate a pressure level that exceeds a pre-set limit. This limit is set to a pressure that is less than the safe operating pressure of the pipeline 4. The use of two failsafe HIPPS valves, three pressure transducers and failsafe DCVs ensures high integrity of the system.
Employment of a HIPPS system not only reduces installation costs, but also allows for reduced rating of downstream equipment, such as risers, separators and process plant and permits high pressure fields to be commercially viable by allowing them to tie into existing infrastructures.
Although such known HIPPS systems are effective at protecting the pipeline, they are relatively basic, and do not address the problem of reducing the overpressure in the fluid system, which should be reduced before the HIPPS valves are re-opened and normal operation resumed. It is of course preferable that the overpressure is reduced in a controlled manner which minimizes the risk of component damage.